This invention relates to a process for forming cermet thin films in desired resistance patterns, and more particularly, to such a process that provides improved reliability and higher resolution.
Cermet is a resistive compound of silicon monoxide and chromium, or silicon an chromium. Cermet is used to form thin film resistors on monolithic circuits, hybrid circuits, or resistor networks used in conjunction with monolithic and hybrid circuits. In general, cermet is etched to form patterns using either a hydrofluoric-acid-based etchant or lift-off techniques. Several problems are encountered in both of these etching methods.
One of the difficulties associated with the use of hdrofluoric-acid-based etching is that the hydrofluoric acid is not a preferential etchant. It etches the silicon dioxide layer which serves as a diffusion mask and an insulator, as well as the cermet, giving rise to pin holes in the silicon dioxide layer or removal of areas thereof. If the pattern of cermet happens to be deposited on the silicon dioxide layer, the silicon dioxide is etched out from under the cermet, leaving the cermet on a pedestal. Another difficulty is that the photoresist that is used to pattern the cermet does not adhere well to the cermet. Because of the poor adherence, the photoresist is undercut, and this gives rise to resolution problems.
The lift-off technique involves etching a pattern in an underlayer such as copper or aluminum and then depositing cermet over the patterned layer so as to fill the voids and cover the patterned layer. A preferential etchant dissolves the supporting underlayer and causes the unsupported cermet to tear away from the cermet-filled voids. The lift-off technique is difficult to control because it involves etching small areas out of a larger field. There is also the problem of providing good adherence of the cermet to the lift-off material underlayer, in that any cleaning treatment of the lift-off material that is effective to provide good adherence of the cermet inevitably results in damaging the lift-off material. Yet another difficulty is posed by requiring the lift-off material to be etched out from under the cermet. The cermet tends to protect the lift-off material from the etchant, with the result that the lift-off material is unetched in many areas and the cermet tears away roughly from these areas and leaves ragged edges.